Passive radiators, also known as drone cones or assisted-bass resonators, have been commercially available for over 37 years, as noted in the 1954 Journal of Audio Engineering Society, by Harry F. Olson et al. (Vol. II, No. 4, p. 219). A passive radiator loudspeaker system is a direct radiator system that uses an enclosure with a driven loudspeaker and an undriven suspended diaphragm, similar to the diaphragm of the driven speaker. The term "driven loudspeaker" refers to a cone and diaphragm assembly actuated by an electromagnetic signal to move air and, thus, produce sound. In contrast, an "undriven" or passive radiator consists only of a cone and diaphragm and does not include an electromagnetic activator or driver. However, a passive radiator's diaphragm and cone are moved in a secondary or passive response by the air pressure variation in the enclosure produced by the movement of the driven loudspeaker cone.
The principle use for the passive radiator is to replace the mass and stiffness of the air in a vented loudspeaker with a mechanical equivalent in a sealed enclosure that requires less enclosure volume than a vented system. A passive radiator, thus, substantially reduces the size of the enclosure that is required for a loudspeaker system while obtaining tuning equivalent to that achieved by a vent.
Loudspeakers, or powered speakers, require vents, or ports, to accommodate the varying air volume by enabling air pressure to be released in the loudspeaker enclosure, which have been produced by the oscillations of the cone of the driven loudspeaker. A large diameter vent requires considerable length, and therefore, a larger enclosure to house the vent. For example, a typical eight-inch, two-way loudspeaker system has a front baffle size of approximately 10 by 15 inches. To obtain linear low frequency power response from a vented loudspeaker system, the vent area must be at least one-half the diaphragm area. To meet this requirement, the size of the baffle that is used must be increased, Also, the vent must be properly sized such that it is large enough to function without creating vent noise, but small enough to minimize the physical size of the speaker enclosure. If the vent is too small, air velocity through the vent increases causing vent noise. Small vents also suffer from high turbulence during reproduction of musical material and/or sound that includes any loud or low bass content. In addition, sizing a vent too small causes vent power compression, which results in the loss of low frequency output from the loudspeaker system.
FIGS. 13A-13D illustrate some typical vent openings of varying size relative to a cone area of the speaker used. In FIG. 13A, the speaker 2 has a diaphragm or cone area A.sub.0, and three vent sizes 4,6, and 8, which are shown in FIGS. 13B-13D, respectively. Vent 4 has an area of 0.1 A.sub.0, vent 6 has an area of 0.25 A.sub.0, and vent 8 has an area of 0.5 A.sub.0. Lines 10, 12, 14, 16 and 18 as shown in graph FIG. 14 illustrate the relative corresponding frequency response for the different vent sizes of FIGS. 13A-13D for both large and small signals (i.e., one watt and 100 watts). Line 10 is a small signal response curve for all vents. Line 12 is vent area A.sub.0, line 14 is vent area 0.5 A.sub.0, line 16 is vent area 0.15 A.sub.0, and line 18 is vent area 0.1 A.sub.0. Based on the graph shown in FIG. 14, it will be apparent that a small vent causes a reduction in low frequency output at high power levels. Thus, vent power compression should be avoided to achieve reasonable acoustic output at low frequencies.
A passive radiator is beneficial in that a smaller speaker enclosure can be used. This is because the passive radiator provides a mechanism that is a substitute for vents, while consuming less volume. At low frequencies, a passive radiator diaphragm, which is the key component of the passive radiator, moves in response to pressure (sound) variations in a sealed speaker enclosure in a manner similar to the movement of a mass of air through the vent in a vented system. Because of the similarity of a passive radiator to a vent, a passive radiator performs like a properly sized vent if the passive radiator has sufficient linear excursion (length of travel), does not exhibit diaphragm breakup, and there is sufficient compliance (also known as suspension).
There are both technical and marketing reasons to use a passive radiator in a loudspeaker system. Recently, speaker systems employing passive radiators have become popular due to marketing efforts, rather than for technical reasons. When a passive radiator is used in a system, it appears as if the speaker system has more speakers. Usually, a passive radiator is the same size as the driven speaker, and from outward appearance, looks very similar to the driven speaker. As mentioned previously, however, the passive radiator does not have a voice coil and magnet assembly (i.e., it does not include a driver assembly). The purpose of a passive radiator is to serve as substitute for a vent. This enables the use of a smaller speaker enclosure for equivalent low frequency performance. Because the speaker enclosures have become smaller, many users place the speakers on a bookshelf, which takes up less room than traditional large speakers.
An important consideration for proper functional operation of a passive radiator is that it exhibits true pistonic motion over its entire design frequency range, and accommodate a very large linear excursion. Pistonic motion means that the entire diaphragm and suspension (compliance) move back and forth to displace air substantially the same distance, in the same direction, at the same time. This movement replicates the reciprocal movement of a piston. Linear excursion refers to the length of travel of the radiator assembly. Both effective pistonic motion and relatively large linear excursion are very difficult to achieve with conventional passive radiator technology.
FIG. 1 shows a cut-away of a conventional prior art passive radiator consisting of a cone 20, and an outer means 22 for suspending the cone (also referred to as a compliance), which is attached to the back surface 24 of cone 20. Conventional passive radiator also includes a dust cap 26, a singular spider 28, which supplies most of the mechanical restoring force to cone 20, a voice coil form 30, and a mass ring 32 that comprises additional mass used to tune the system. FIG. 2 shows an alternative method for attaching compliance 22 over the front surface 34 of cone 20 such that compliance 22 extends between cone 20 and a mounting gasket 36. The speaker frame is not shown in either of these figures.
Typical passive radiators have problems with diaphragm breakup, or non-pistonic motion, as illustrated in FIGS. 7A and 7B. FIG. 7A shows a standing wave of one degree of freedom across the diaphragm. One full wave length is shown. Peak displacement of the standing wave occurs at 38 and 40 in FIG. 7A. Standing waves 38 and 40 are shown with maximum peak amplitude at points "B" and minimum peak amplitude at points "A". FIG. 7B is the plan view of the standing wave breakup phenomena illustrated in FIG. 7A. Passive radiator designs should minimize or eliminate the development of such standing waves.
Another serious design challenge in passive radiators is minimizing the many different breakup modes of the outer compliance, which produces undesirable audible effects. FIG. 8A illustrates the ideal performance of the outer compliance for both inward and outward displacements. In FIG. 8A, all points on the compliance rim move together. FIG. 8B shows the effects of compliance breakup in a rim resonance mode of operation, during large excursions. In some cases, the compliance will actually move out of phase for in the opposite direction), relative to the radiator's diaphragm.
Clearly, a new passive radiator is needed that will reduce the inherent problems of typical passive radiators to below the threshold of audibility. The new passive radiator would thus permit large excursions and will minimize breakup in either the diaphragm or compliance.